This invention relates to means for advancing strip material in both directions along its length and particularly, strip material having holes or perforations spaced at intervals along its length, often in one or both margins of the material. Such means are utilized, for example, in machines for producing paper business forms and in the business machines such as high speed printers with which such forms are used. Motion picture cameras and projectors employ means of this kind in their film transport mechanisms.
An example of means of this kind is an endless mechanism including a toothed flexible belt trained on sprockets and carrying pins for engaging simultaneously a number of successive perforations in paper business forms. Such means are sometimes referred to as tractor mechanisms and such belts, as tractor belts. See, for example, U.S. Pat. No. 3,507,431.
This invention also includes a particular method for making the article embodying the invention.
Applications utilizing tractor mechanisms to advance and position continuous strip material typically involve stop and start motion and require exact control of the material being handled. Accurate positioning of successive segments or lines of the material at predetermined stations is necessary, for example, when moving paper business forms through high speed printers to insure correct placement of the printed material on the form. Even more critical is the positioning of each successive frame of a motion picture film in the projection aperature or gate. Failure to bring each frame to the same position results in undesirable "jump" of the projected image. In sophisticated applications such as scientific and medical projectors, projected images of successive frames of film on perceivably different spots on a viewing screen is unacceptable.
Other demands on tractor mechanisms and the like include the need to accelerate and decelerate the material at high rates when starting, moving, and stopping it to bring successive segments into registry with a predetermined station or position within a short interval of time. Simultaneous engagement of a number of perforations in the material being moved by pins or the like of the tractor mechanism is helpful in distributing the high inertia loads involved in providing such motions over a length of material and a number of perforations and thereby avoiding damage to the perforations in it. Mutilation and damage to the perforations is undesirable because it tends to prevent positive engagement of them by the tractor mechanism resulting in inaccurate control and movement of the material through its desired motion.
The foregoing problems involved in advancing perforated strip material dictate tractor mechanisms having pins or other means for engaging the perforations of the material that are accurately spaced at intervals corresponding closely to the spacing of the perforations, pins which fill the perforations without deleterious distortion of them to reduce play in their engagement with the pins, and pins which do not damage or degrade the perforations by wear and tear and which, in turn, are not themselves altered significantly by wear on the material being advanced.
Various endless belt, chain and band constructions have been utilized in the past. They have shortcomings of one type or another, however. Some fail to maintain a constant pitch diameter and resultant accurate pin spacing; others are difficult to fabricate consistently to the tolerances required for acceptable operation or are so expensive as to be impractical, and others are subject to inaccuracy through wear inherent in their construction materials.